Passive Infra-Red (PIR) motion detectors, also referred to as intrusion or intruder detectors, are configured to detect movement of infra-red radiation emitted by the human body within a protected space. The human body typically gives off infra-red radiation at wavelengths ranging from 6 to 15 μm. Such a detector typically includes an infra-red sensor element and an array of Fresnel lenslets or mirrored segments window opening to focus incident infra-red radiation onto the sensor element. The optics collect IR light from specific zones within the protected space, and movement into and out of zones causes an amount of IR light detected to vary in time. A signal processing electronic component operates to process a signal output by the sensor element to detect an intrusion, and to trigger an alarm. In conventional PIR detectors, the sensor measures changes in IR light intensity and not the absolute level.
Anti Masking detection, using Active Near Infra Red (NIR, at the wave length of less than 1 μm) is an added technology to PIR detectors that monitors the PIR lens performance (i.e. PIR Lens transparency) and also may check the presence of close proximity objects (up to a distance of 1 m) for the purpose of alerting in case of unit optical masking. A review of some anti-masking technologies is found in White Paper document F01U075615-01, 2008, by Bosch Security Systems, Inc, Fairport, N.Y., USA. Causes can be due to blocking material applied on the unit lens surface intentionally, for the purpose of sabotaging the unit operation, unintentionally, such as a box placed near the unit lens, or degradation of the optics due to heavy accumulation of dirt or dust.
There are many types of Passive Infra-red blocking materials, however there are some materials which are substantially transparent at visual and NIR wavelengths but are opaque for infra-red wavelengths at which the human body radiates. For example, hair spray may be transparent at visual and NIR wavelengths but is opaque at PIR infra-red wavelengths. The problem is that a PIR motion detector can be deliberately sprayed from a distance with such a blocking substance, such as hair spray, which disables the unit operation by substantially attenuating the entrance of collected infrared light into the unit thus preventing it from detecting movements. Such visually transparent blockers may be sprayed during a period when the unit is unarmed, at a distance far enough from the unit so the proximity of the person or spray can will not be detected by the unit proximity protection, and after its application onto the lens surface, are hard to be noticed. Such material is also difficult to detect by a NIR lens transparency measurement since it may affect NIR light intensity by only a few percent.
Anti-Masking detectors are known in the art. In U.S. Pat. No. 7,884,313 and in US patent publication 2009/0302222, a waveguide optical structure is proposed that lies at the surface of a PIR lens to be able to detect when a coating is applied to the lens, since the coating will adversely affect TIR at the surface of the optical structure. These structures are separate from the Fresnel lenslet body of the PIR detector and require the use of additional dedicated pair of NIR light source and NIR sensor.
In U.S. Pat. No. 5,942,976, a light source is located external to the IR transparent detector window that encloses internal focusing optics, a large area diffraction grating is provided on the cover, and a light detector is provided inside the detector housing to sense the light from the light source. In U.S. Pat. No. 4,709,153, an external light beam is directed onto a sensor through an IR transparent window. Coating the cover with an IR opaque material can be detected by the absolute drop in light received at the light detector inside the housing. In EPO patent application publication 0499177, a wide external light beam is arranged to direct light onto an internal sensor for the external beam, such that coating of the Fresnel lens and placing a screen in front of the detector can be detected as a result of reflection of external beam light off the screen and into the detector.